1. Field of the Invention
The present invention relates to an optical sheet and an optical sheet lamination ("lamination" includes stacking or putting on in not contact with another) composed of a prism sheet, a lens sheet and/or an optical diffusing sheet suitable for use in a back light surface light source used in illuminating from the back a light-transmissive type display panel of a light-transmissive type liquid crystal display apparatus, an advertising board or the like, and a surface light source device and a light-transmissive type display apparatus using the same optical sheet or optical sheet lamination.
2. Prior Art
In a liquid crystal display apparatus in recent years, according to necessity of reducing of its power consumption and making it thin and light in shape and weight, a surface light source for illuminating the liquid crystal display apparatus from the back has been naturally required also to be made thin and light in shape and weight and has been reduced in power consumption in its light source by effectively utilizing light from the light source for reduction of power consumption.
In response to such requirement, such techniques for directing light from a surface light source in a particular direction (in the normal direction to a light outputting surface in many cases) have been proposed as disclosed in Japanese laid-open publication Tokkaisho No. 60-70601, Tokkaihei No. 2-84618, Jikkaihei No. 3-69184, Tokkaihei No. 7-191319 and the like.
As a surface light source device used in a light-transmissive type liquid crystal display apparatus and the like there are light source devices of an edge-light type and of a direct back light type.
A surface light source device of an edge-light type, as disclosed in a Japanese laid-open publication Tokkaihei No. 3-5725, has a light-source light inputted into it usually through one side end surface of a plate-shaped light guide means of a transparent acryl resin or the like, leads the light to an light outputting surface which is the other surface of the light guide means, and therefrom outputs the light to the back surface of a liquid crystal panel or the like.
In such a case, in order to improve the efficiency of utilizing light, an optical reflector plate or an optical reflector film is often provided on the opposite surface of said light guide means to the light outputting surface and in order to make the output light uniform, a light diffusing sheet having a light diffusing function is often provided at the light outputting surface side of the light guide means, as disclosed in a Japanese laid-open publication Jikkaihei No. 5-73602 and the like, for example.
A surface light source device of a direct back light type, as disclosed in a Japanese laid-open publication Jikkaihei No. 2-33001 for example, generally reflects a light-source light onto the back surface of a liquid crystal panel or the like by means of a reflector plate and diffuses its output light by means of a light diffusing sheet disposed at the light outputting surface side so that the shape of the light source cannot be identified by human eyes.
Some of such surface light source devices of an edge-light type or a direct back light type as described above are provided with a prism sheet (prism film) or a lens sheet (lens film) which has an array of plural unit prisms or plural unit lenses arranged on the obverse surface of the light-transmissive base material of it in order to output light from a surface light source collectively in a particular direction, as described above.
Various applications of such optical sheets have been proposed in combination of plural optical sheets and in direction of setting the side at which unit prisms or unit lenses are formed (prism side or lens side) with regard to the light source side.
In such an optical sheet as described above, the opposite surface (reverse surface) to the prism surface or lens surface is often formed into a flat and smooth surface.
In an optical sheet as described above, a pattern where bright and dark parts are alternately repeated by light from a surface light source is observed in case of combination of a light guide means, a diffusing sheet, another optical sheet and the like, and a problem that an image composed of pixels is disarranged has been caused by this in case that such an optical sheet is used in a liquid crystal display apparatus, for example. On the other hand, as disclosed in a Japanese laid-open publication Tokkaihei No. 7-151909 for example, a method for solving this problem has been proposed on the assumption that said pattern of repeating bright and dark parts is caused by interference fringes generated by an outside light source between the prism surface of one prism sheet and the flat and smooth surface of the other prism sheet in case of using two prism sheets, for example.
When we have checked this assumption, however, a pattern of repeating bright and dark parts has been observed in a surface light source device even in a darkroom where no light enters from any outside light source.
That is to say, we have confirmed that interference fringes are generated by a surface light source rather than an outside light source and further generated between the flat and smooth surface of a prism sheet and the flat and smooth face of a light guide plate or the flat and smooth surface of a diffusing plate or other prism sheets.
On the other hand, a method of forming minute rugged parts satisfying a particular condition on the flat and smooth surface of a prism sheet is also conceivable as disclosed in said Tokkaihei No. 7-151909, but in this case, there is a problem that the primary function of a prism sheet for improving the brightness by collecting light from a surface light source in a particular direction, for example, in the normal direction to the light outputting surface is deteriorated.
As for the light outputting side, many existing prism sheets as described above are made so as to cover defects (flaws, stains and the like) of a prism sheet by laminating a light diffusing sheet on the prism surface side which is a light outputting surface and are somewhat weakened in directivity of their light outputting characteristics.
Such a prism sheet having a light diffusing sheet laminated on it has a problem that a prism (or lens) part touching the light diffusing sheet is damaged by a rugged part on the surface of the light diffusing sheet and this flaw deteriorates a uniform and flat light-emitting state as a whole.
On the other hand, although a method of forming the prism sheet side of a light diffusing sheet into a flat and smooth surface in the same way as the light inputting surface of said prism sheet is also conceivable, such a method as this has not come to be in practical use since there is a problem that interference fringes or moire patterns are generated between the flat and smooth surface of said light diffusing sheet and the flat and smooth surface of a light guide plate or the flat and smooth surface of a prism sheet.